1. Field of the Invention
This invention relates to a screen printing method and an apparatus therefor, and more particularly to a screen printing method which is adapted to print a pattern on an object to be subjected to printing such as a substrate for a printed wiring board or the like and an apparatus therefor.
2. Description of the Prior Art
Conventionally, screen printing has been typically carried out using a screen printing apparatus including a single printing head provided with a single doctor plate (ink return) and a single squeegee in such a manner as shown in FIGS. 1 and 2. More particularly, FIG. 1A shows a state wherein a paste 20 on a screen 22 has been returned to a leftmost position on the screen 22 adjacent to a squeegee 24 by means of a doctor plate 26. Then, the squeegee 24 is lowered to downwardly force the screen 22 against an object to be subjected to printing (hereinafter referred to as "printed object") 28 (FIG. 2) such as a substrate for a printed wiring board made of alumina or the like as shown in FIG. 1B. Thereafter, the printing head is laterally moved to cause the squeegee 24 to be moved laterally or in a right-hand direction while pressing the screen against the printed object and moving the paste as shown in FIGS. 1C and 1D, resulting in printing on the printed object.
More particularly, as shown in FIG. 2 which enlargedly shows a portion of FIG. 1C surrounded by dashed lines, movement of the squeegee 24 in a direction indicated at an arrow while pressing the screen 22 against the printed object 28 causes the paste 20 to be printed on the printed object or substrate 28 via fine through-holes 30 constituting a printing pattern of the screen 22, resulting in a printed pattern being formed on the substrate 28.
Thus, when the squeegee 24 is moved to a rightmost position on screen 22 as shown in FIG. 1D, the squeegee 24 is raised to be separated from the screen 22 as shown in FIGS. 1D and 1E and then returned to the original position as shown in FIG. 1E and 1F. Simultaneously, the doctor plate 26 transfers the paste 20 to the leftmost position on the screen 22 as shown in FIG. 1F. Simultaneously, the squeegee 24 is returned to the original position.
Unfortunately, in the conventional screen printing described above, the doctor plate 26 merely acts to return the paste 20 toward the squeegee 24, accordingly, the squeegee 24 transfers the paste 20 while carrying out printing of the paste 20 on the board 28. This causes the paste 20 transferred with movement of the squeegee 24 to be varied in an amount as it is transferred, so that the thickness of a film of the paste 20 printed on the screen 22 is varied and the paste 20 is excessively deposited on the substrate 28. Such disadvantages substantially interfere with fining of a printed pattern on the substrate and formation of a fined pattern.
The conventional screen printing has another disadvantage in that the printing pressure which the printed object or substrate 28 receives is gradually increased as the printing operation is repeated, because tension of the screen 22 is decreased with repetition of the printing operation even when squeegee pressure at which the squeegee 24 presses the screen 22 and substrate 28 is kept constant in the situation wherein the squeegee 24 presses the screen 22 against the substrate 28 to deposit the paste 20 onto the substrate 28 while transferring the paste.
Further, since conventional screen printing fails to directly monitor the printing pressure, it is conventional to replace the screen when it is deteriorated to a degree sufficient to decrease its tension.
However, it is presently highly to fine a printed pattern or form a fine pattern. This requires a thickness of a printed pattern to be rendered more uniform. For this purpose, it is required to keep the printing pressure within a predetermined range. Unfortunately, conventional screen printing does not meet the requirements because it fails to directly control the printing pressure as described above.
Furthermore, the conventional screen printing requires that aligning of the screen be carried out at every replacement of the screen. More specifically, proof printing must be carried out at every replacement of the screen in order to determine whether a printing pattern formed on the screen is properly positioned with respect to the substrate. As a result when the positioning is improper, it is necessary to carry out positional correction of the screen and substrate in relation to each other. Thus, the aforementioned conventional screen printing is highly troublesome and requires much time and labor.